This research will investigate the thoracic chemoreceptor (aortic bodies) of adult dogs. It is particularly aimed at those group 4 bodies which lie between the aorta and pulmonary artery which have a blood supply from a small artery which is a branch of the common left coronary artery or one of its major branches. The origin of the small vessel will be determined by the injection of 5 micron g doses of serotonin or phenyl biguanide through an exploring intracoronary catheter to locate the point of maximal reflex increases in arterial pressure and heart rate. The common left coronary artery will be cannulated with a silicone rubber tipped cannula which does not require dissection. The left coronary artery will be perfused with blood at various pressures and levels of PO2, PCO2, and pH. We will investigate the role of the small vessel and of the aortic nerves in the reflex depressor response which follows elevation of pressure in the left coronary artery and of possible other reflexes initiated by alteration of gases in the perfusing blood. We will also attempt to determine whether there are differences in the function of the other two groups of chemoreceptors.